Method for forming a black-passivation layer on a zinc-iron alloy and black- passivation composition

ABSTRACT

The present invention relates to a method for forming a black-passivation layer on a zinc-iron alloy of a substrate, a black-passivation composition for depositing a black-passivation layer on such, in which the black-passivation composition includes one or more than one blackening agent selected from the group consisting of formula (I) and formula (II) as described hereinafter, and a respective use of said blackening agents for blackening a zinc-iron alloy.

FIELD OF THE INVENTION

The present inventions refers to a method for forming ablack-passivation layer on a zinc-iron alloy and a black-passivationcomposition for depositing a black-passivation layer on such, whereinthe black-passivation composition comprises one or more than oneblackening agent selected from the group consisting of formula (I) andformula (II) as described hereinafter.

BACKGROUND OF THE INVENTION

To protect metallic substrates from corrosive environmental influences,different methods are available according to the prior art. To apply aprotective coating/layer of a metal or metal alloy on the metallicsubstrate is a widely used and established method. A well know principleis the deposition of a zinc or zinc-nickel coating/layer on metallicsubstrates, such as iron metal substrates. Such coating/layers are oftencalled conversion coatings/conversion layers. Such conversioncoatings/conversion layers typically comprise reaction products (whichare insoluble in aqueous media over a wide pH range) of the metallicsubstrate with a respective conversion treatment solution. In order tofurther increase the corrosion resistance, such conversioncoatings/conversion layers are additionally passivated with apassivation layer by contacting it with a passivation composition. Suchpassivation compositions and respective methods are known in the art.

Besides a zinc and zinc-nickel coating/layer, other alloys are becomingmore prominent such as zinc-iron. Since nickel becomes more and morequestionable in regard to environmental and health issues, less criticalalternatives are desired.

In many cases the passivation composition furthermore modifies the colorof the conversion coating/conversion layer, for example into a bluish oreven dark black color. Depending on the application, such a colormodification is often very much desired for optical reasons, inparticular in the automotive field.

However, for certain conversion coatings/conversion layers a suitablecolor modification caused by means of passivation compositions is eithernot available at all, provides an insufficient color modification,deteriorates corrosion resistance upon color modification, or requiressophisticated passivation compositions, which are demanding to handle.This is particular true for zinc-iron conversion coatings/conversionlayers, e.g. substrates protected with a zinc-iron layer. Blackening ofsame without compromising the corrosion resistance is still a demand.

EP 1 816 234 B1 refers to an aqueous passivating coating composition forzinc or zinc alloys and method for using same.

CN 104651823 A refers to a cobalt-free, environmentally friendlytrivalent chromium black passivation liquid, comprising tungstendisulfide particles as blackening agent. CN′823 is silent with respectto zinc iron alloys.

WO 97/13888 A1 refers to a non-chromate containing, corrosion-inhibitingcoating composition capable of protecting a wide variety of metalsurfaces.

WO 02/49960 A2 refers to specific tripolyphosphates, mixtures thereof,and their use as anti-corrosion agents and as biocides (anti-microbialagents) and, where applicable, as encrustation-inhibiting agents.

EP 3 360 989 A1 refers to a method for electrolytically passivating anoutermost chromium or outermost chromium alloy layer to increasecorrosion resistance thereof.

US 2004/0170848 A1 refers to a corrosion inhibiting composition forcoating an article or substrate such as a metal, metal coating,chromated metal coating, and the like comprises a film-forming compoundsuch as a wax or a polymer, and a sulfide salt or thio compound or aderivative of a thio compound. US′848 is silent with respect toblackening.

Although black-passivation compositions are described in the art, thereis an ongoing demand to improve blackening of zinc-iron conversioncoatings/conversion layers.

Objective of the Present Invention

It was therefore the objective of the present invention to provide amethod for forming a black-passivation layer particularly on a zinc-ironalloy with improved blackening quality and without compromisingcorrosion resistance. Furthermore, a respective black-passivationcomposition is needed too.

It is furthermore the objective to overcome the disadvantages mentionedabove and to provide in particular a method (along with a respectivecomposition), which is easy to handle and avoids sedimentation by beingsubstantially free of particles.

SUMMARY OF THE INVENTION

The objectives mentioned above are solved by a method for forming ablack-passivation layer on a zinc-iron alloy, the method comprising thesteps:

-   (a) providing a substrate comprising the zinc-iron alloy,-   (b) providing a black-passivation composition for depositing the    black-passivation layer on the zinc-iron alloy, the composition    comprising    -   (i) one or more than one blackening agent selected from the        group consisting of    -   formula (I),

-   -   -   wherein            -   R¹ and R² are independently selected from the group                consisting of hydrogen and C1 to C5 alkyl,            -   R³ is selected from the group consisting of sulfonic                acid, carboxylic acid, alkyl carboxylic acid, phosphonic                acid, salts and esters thereof, and            -   n is 1, 2, 3, 4, or 5,

    -   and formula (II),

-   -   -   wherein            -   R⁴ is selected from the group consisting of sulfonic                acid, carboxylic acid, alkyl carboxylic acid, phosphonic                acid, salts and esters thereof, and            -   m is 1, 2, 3, 4, or 5, and

    -   (ii) one or more than one species of metal ions selected from        the group consisting of trivalent chromium, titanium, and        zirconium, and

-   (c) contacting the substrate with said black-passivation composition    such that the black-passivation layer is formed on the zinc-iron    alloy.

By utilizing said one or more than one blackening agent, a very goodblackening of a zinc-iron alloy is obtained along with good corrosionresistance. Moreover, the method of the present invention is simple andcan be easily carried out. Our own experiments have furthermore shownthat the blackening obtained by means of the method of the presentinvention and the respective black-passivation composition of thepresent invention is highly specific for zinc-iron alloys. Ownexperiments confirm that zinc alone and zinc nickel alloys are notblackened. Further details are given in the examples section below inthe text.

The present invention also concerns a respective black-passivationcomposition as further described below in the text as well as arespective use of said one or more than one blackening agent forblackening a zinc-iron alloy. Generally, features described in regard tothe method of the present invention, in particular features described asbeing preferred, apply likewise to the black-passivation composition ofthe present invention, most preferably to a black-passivationcomposition described as being preferred, and apply likewise to the useaccording to the present invention, most preferably to the use describedas being preferred.

DETAILED DESCRIPTION OF THE INVENTION

In the context of the present invention, ions of trivalent chromiumrefers to chromium ions with the oxidation number+3 (also calledtrivalent chromium ions) including the free and complexed form,respectively. Thus applies mutatis mutandis to the metal ions oftitanium and zirconium, respectively.

In the context of the present invention, the term “black-passivationlayer” also denotes a black-conversion layer.

The Substrate

Preferably, the black-passivation layer has a darkness value L* of 40 orbelow, preferably of 33 or below, most preferably of 25 or below, basedon the CIELAB color space definition.

As mentioned above, the method of the present invention is highlyspecific for a zinc-iron alloy. Preferred is a method of the presentinvention, wherein in the zinc-iron alloy the amount of iron ranges from0.1 wt.-% to 30 wt.-%, based on the total weight of the zinc-iron alloy,preferably 0.6 wt.-% to 28 wt.-%, more preferably 2.1 wt.-% to 25 wt.-%,even more preferably 3.5 wt.-% to 22 wt.-%, most preferably 4.9 wt.-% to18 wt.-%, even most preferably 6.1 wt.-% to 15 wt.-%. A very preferredamount of iron ranges from 4.9 wt.-% to 30 wt.-%. In this very preferredrange, an excellent blackening is easily obtained.

The method of the present invention generally applies to a zinc-ironalloy. Preferred is a method of the present invention, wherein thezinc-iron alloy is present on the substrate as a layer, preferably as alayer resulting from a galvanization process, most preferably from azinc-iron galvanization process. Thus, most preferred is a method of thepresent invention, wherein the zinc-iron alloy is distinct from the restof the substrate. In such a way, the substrate is typically protectedfrom corrosion.

Preferred is a method of the present invention, wherein the substratecomprises iron. This means that the substrate preferably comprises abase material, preferably a ferrous base material, more preferablysteel, on which the zinc-iron alloy is deposited. Thus, the zinc-ironalloy is distinct from the rest of the substrate (i.e. is represented bythe base material).

However, in a few cases, a method of the present invention is preferred,wherein the substrate comprises the zinc-iron alloy in a sense that thesubstrate itself is made of a zinc-iron alloy. In other words,preferably the base material is already the zinc-iron alloy and thus,the base material is the substrate.

Preferred is a method of the present invention, wherein the substrate isa metal or metal alloy substrate, preferably the substrate comprisesiron, most preferably the substrate comprises iron and is different fromthe zinc-iron alloy.

Preferred is a method of the present invention, wherein the substrate isa work piece requiring anodic corrosion resistance, most preferablyrequired due to environmentally caused corrosion.

Typically, a preferred substrate is selected from the group consistingof screws, bolts, nuts, and automotive parts.

Preferred is a method of the present invention, wherein more than onesubstrate is provided in step (a), preferably a plurality of substratesis provided in step (a). This in particular applies if the substraterefers to screws, bolts, and nuts.

Preferred is a method of the present invention, wherein the substrate is(preferably the substrates are) provided in a barrel or fixed on a rack.Thus, the method of the present invention is applicable to both kinds.

The Black-Passivation Composition

In the method of the present invention a black-passivation compositionis utilized, preferably the black-passivation composition of the presentinvention (see text further below).

Preferably, the black-passivation composition is also called aconversion composition.

Preferred is a method of the present invention, wherein theblack-passivation composition is aqueous (i.e. comprises water), whereinpreferably water has a concentration of more than 50 vol.-% based on thetotal volume of the black-passivation composition, more preferably of 75vol.-% or more, most preferably of 90 vol.-% or more. Very preferably,water is the only solvent.

Preferably, the black-passivation composition is a solution. Thus,preferred is a method of the present invention, wherein theblack-passivation composition is substantially free of, preferably doesnot comprise, particles (including colloids).

Preferred is a method of the present invention, wherein theblack-passivation composition is acidic, preferably having a pH from 1.0to 4.5, preferably from 1.2 to 4.0, more preferably from 1.4 to 3.3,even more preferably from 1.5 to 2.8, most preferably from 1.6 to 2.2.

As mentioned above, the black-passivation composition comprises one ormore than one blackening agent as defined above.

As indicated by formula (I) and (II), respectively, the one or more thanone blackening agent utilized in the black-passivation composition is anorganic blackening agent. Preferred is a method of the presentinvention, wherein the black-passivation composition is substantiallyfree of, preferably does not comprise, an inorganic blackening agent. Aninorganic blackening agent is for example disclosed in CN 104651823 A.

Preferred is a method of the present invention, wherein theblack-passivation composition comprises at least one or more than one(preferably one) blackening agent of formula (I). According to ownexperiments, a blackening agent of formula (I) provides excellentresults (see examples below).

Only in some cases, a method of the present invention is preferred,wherein the black-passivation composition is substantially free of,preferably does not comprise, a blackening agent of formula (II). Thismost preferably applies if the black-passivation composition comprisesalready a blackening agent of formula (I).

Generally preferred is a method of the present invention, wherein in theblack-passivation composition the one or more than one blackening agentselected from the group consisting of formula (I) and formula (II) arethe only blackening agents in the black-passivation composition.

Preferred is a method of the present invention, wherein in theblack-passivation composition the one or more than one blackening agenthas a total concentration ranging from 0.2 mmol/L to 100 mmol/L, basedon the total volume of the black-passivation composition, preferablyfrom 0.3 mmol/L to 80 mmol/L, more preferably from 0.4 mmol/L to 60mmol/L, even more preferably from 0.8 mmol/L to 45 mmol/L, mostpreferably from 1.6 mmol/L to 38 mmol/L.

In particular preferred is a method of the present invention, whereinthe black-passivation composition comprises one or more than one(preferably one) blackening agent of formula (I) in a totalconcentration ranging from 0.4 mmol/L to 25.0 mmol/L, based on the totalvolume of the black-passivation composition, preferably from 0.6 mmol/Lto 20.0 mmol/L, more preferably from 0.8 mmol/L to 12.0 mmol/L, evenmore preferably from 1.0 mmol/L to 10.0 mmol/L, most preferably from 1.2mmol/L to 8.0 mmol/L. Most preferably, in combination with such a totalconcentration, compounds of formula (I) are the only blackening agentsin the black-passivation composition utilized in the method of thepresent invention. Surprisingly, an excellent blackening was obtainedeven with a comparatively low total concentration of compounds offormula (I) including a total concentration of 0.4 mmol/L. It is verydesired to maintain a comparatively low concentration of blackeningagents such that the life-time of a respective black-passivationcomposition is as long as possible. A very preferred total concentrationranges from 0.4 mmol/L to 8.0 mmol/L.

Preferred is a method of the present invention, wherein R¹ and R² areindependently selected from branched and unbranched C1 to C5 alkyl,preferably unbranched C1 to C5 alkyl.

Preferred is a method of the present invention, wherein in R³ and R⁴ thesalts are independently selected from the group consisting of ammoniumsalts and alkaline salts, preferably from the group consisting ofammonium, sodium, and potassium.

Preferred is a method of the present invention, wherein R¹ and R² areindependently selected from the group consisting of hydrogen, methyl,ethyl, 1-propyl, 2-propyl, 1-butyl, 2-butyl, and tert-butyl, preferablyhydrogen, methyl, ethyl, 1-propyl, and 2-propyl, most preferablyhydrogen and methyl.

Preferred is a method of the present invention, wherein at least one ofR¹ and R² is an alkyl group, preferably as described above as beingpreferred.

Preferred is a method of the present invention, wherein n is 2, 3, or 4,preferably 3.

In the method of the present invention, R³ is selected from the groupconsisting of sulfonic acid, carboxylic acid, alkyl carboxylic acid,phosphonic acid, salts and esters thereof.

Sulfonic acid preferably means —SO₃H, wherein “˜” denotes the covalentbond connecting this group with the rest of the compound of formula (I).Carboxylic acid preferably means ˜COOH, wherein “˜” denotes the covalentbond connecting this group with the rest of the compound of formula (I).Alkyl carboxylic acid preferably means a saturated, branched orunbranched alkyl comprising one or more than one (preferably two)carboxylic acid groups, more preferably means a saturated, branched orunbranched C1 to C6 alkyl (preferably C2 to C4 alkyl) comprising one ormore than one (preferably two) carboxylic acid groups, even morepreferably it comprises —C(COOH)(CH₂)_(k)COOH, wherein “˜” denotes thecovalent bond connecting this group with the rest of the compound offormula (I) and k is an integer ranging from 1 to 5, most preferably itcomprises —C(COOH)CH₂COOH. Phosphonic acid preferably means —PO₃H₂,wherein “˜” denotes the covalent bond connecting this group with therest of the compound of formula (I). Preferably, the above mentionedregarding R³ applies mutatis mutandis to R⁴ in formula (II).

Preferred is a method of the present invention, wherein R³ comprises atleast sulfonic acid, salts and/or esters thereof, preferably R³comprises at least sulfonic acid, salts and/or esters thereof, and n is3.

More preferred is a method of the present invention, wherein R³ isselected from the group consisting of sulfonic acid, carboxylic acid,phosphonic acid, and salts thereof, most preferably R³ comprises atleast sulfonic acid and/or salts thereof, preferably R³ comprises atleast sulfonic acid and/or salts thereof, and n is 3.

Most preferred is a method of the present invention, wherein formula (I)comprises the compound 3-(N,N-Dimethylthiocarbamoyl)-thiopropanesulfonicacid, salts and/or esters thereof, preferably3-(N,N-Dimethylthiocarbamoyl)-thiopropanesulfonic acid and/or saltsthereof.

Preferred is a method of the present invention, wherein m is 2, 3, or 4,preferably 3.

Preferred is a method of the present invention, wherein R⁴ comprises atleast sulfonic acid, salts and/or esters thereof, preferably R⁴comprises at least sulfonic acid, salts and/or esters thereof, and m is3.

More preferred is a method of the present invention, wherein R⁴ isselected from the group consisting of sulfonic acid, carboxylic acid,phosphonic acid, and salts thereof, most preferably R⁴ comprises atleast sulfonic acid and/or salts thereof, preferably R⁴ comprises atleast sulfonic acid and/or salts thereof, and m is 3.

Most preferred is a method of the present invention, wherein formula(II) comprises the compound 3-(2-Benzthiazolylthio)-1-propanesulfonicacid, salts and/or esters thereof, preferably3-(2-Benzthiazolylthio)-1-propanesulfonic acid and/or salts thereof.

The black-passivation composition utilized in the method of the presentinvention comprises one or more than one species of metal ions selectedfrom the group consisting of trivalent chromium, titanium, andzirconium. Although a significant blackening of a zinc-iron alloy can beobserved already even without the presence of these metal ions, i.e. inthe total absence of these and other transition metal ions, said metalions are in particular beneficial in order to obtain a significantcorrosion resistance besides the blackening effect. Although a certaincorrosion resistance is in some cases present, corrosion resistance canbe thus significantly further increased.

Preferred is a method of the present invention, wherein the one or morethan one species of metal ions is selected from the group consisting oftrivalent chromium and titanium. More preferred is a method of thepresent invention, wherein the one or more than one species of metalions at least comprises trivalent chromium, most preferred the one ormore than one species of metal ions is trivalent chromium. Ownexperiments have shown that excellent corrosion resistance results areobtained with trivalent chromium ions.

Preferred is a method of the present invention, wherein the metal ionsof trivalent chromium are from an organic and/or inorganic trivalentchromium ion source, preferably from an inorganic trivalent chromium ionsource. A very preferred trivalent chromium ion source is an organicand/or inorganic trivalent chromium ion source. A preferred organictrivalent chromium ion source is trivalent chromium citrate. A preferredinorganic trivalent chromium ion source is trivalent chromium chloridehexahydrate.

Preferred is a method of the present invention, wherein said one or morethan one species of metal ions has a total concentration ranging from0.1 g/L to 30 g/L, based on the total volume of the black-passivationcomposition and based on the mass of the free ions, preferably from 0.2g/L to 20 g/L, more preferably from 0.5 g/L to 14 g/L, even morepreferably from 0.8 g/L to 10 g/L, most preferably from 1.3 g/L to 6.0g/L. A very preferred total concentration is ranging from 0.1 g/L to 4.0g/L. If the total concentration is significantly below 0.1 g/L, in manycases a particular desired corrosion resistance is not achieved. Incontrast, if the total concentration is significantly above 30 g/L, nofurther benefit is identified and costs are typically inacceptable.

Since trivalent chromium ions are preferred, preferred is a method ofthe present invention, wherein the metal ions of trivalent chromium havea total concentration ranging from 0.1 g/L to 8.0 g/L, based on thetotal volume of the black-passivation composition, preferably from 0.2g/L to 7.1 g/L, more preferably from 0.5 g/L to 6.1 g/L, even morepreferably from 0.8 g/L to 5.0 g/L, most preferably from 1.0 g/L to 3.5g/L. Most preferably, the above mentioned total concentration for metalions of trivalent chromium applies with the proviso that these ions arethe only species of transition metal ions in the black-passivationcomposition.

In the following a number of compounds and ions are listed thatpreferably are not contained in the black-passivation compositionutilized in the method of the present invention. Such compounds/ions areeither not helpful for solving the objectives mentioned in the outlineabove or even have a detrimental effect and are therefore avoided. In afew cases the presence of further compounds/ions is inevitable uponutilizing the black-passivation composition. Generally, it is preferredto utilize a comparatively simple black-passivation composition.

Preferred is a method of the present invention, wherein theblack-passivation composition is substantially free of, preferably doesnot comprise, hexavalent chromium. This includes any compounds and ionscomprising chromium with an oxidation number of (+VI). It is a veryimportant objective that the method of the present invention avoids anyutilization of hexavalent chromium due to its environmental and healthissues.

Preferred is a method of the present invention, wherein theblack-passivation composition is substantially free of, preferably doesnot comprise, thioglycolic acid and salts thereof. However, in somecases a method of the present invention is preferred, wherein theblack-passivation composition comprises thioglycolic acid and/or saltsthereof in addition to said one or more than one blackening agentselected from the group consisting of formula (I) and formula (II), asdefined above. Although own experiments (data not shown) indicate thatthe mere presence of thioglycolic acid and/or salts thereof in manycases also cause a significant blackening, the obtained results are lessgood compared to the effect obtained with the blackening agents offormula (I) and (II).

Preferred is a method of the present invention, wherein theblack-passivation composition is substantially free of, preferably doesnot comprise, nickel ions, preferably is substantially free of,preferably does not comprise, nickel.

Preferred is a method of the present invention, wherein theblack-passivation composition is substantially free of, preferably doesnot comprise, intentionally added zinc ions. If zinc ions are present inthe black-passivation composition they are released/dissolved from thezinc-iron alloy upon utilizing the black-passivation composition. Thus,preferred is a method of the present invention, with the proviso that,if zinc ions are present in the black-passivation composition, they arereleased from the zinc-iron alloy. In other word, if zinc ions arepresent, the source is the zinc-iron alloy. After setting up arespective black-passivation composition no zinc ions are typicallypresent. After starting the method of the present invention, the totalconcentration is very low. Upon utilizing the black-passivationcomposition the total concentration typically increases. Thus,preferably, zinc ions are present in a total concentration of 10 g/L orbelow, based on the total volume of the black-passivation composition,preferably of 8 g/L or below, most preferably of 5 g/L or below, afterstep (C) is carried out multiple times.

Preferred is a method of the present invention, wherein theblack-passivation composition is substantially free of, preferably doesnot comprise, silver ions, preferably is substantially free of,preferably does not comprise, silver.

Preferred is a method of the present invention, wherein theblack-passivation composition is substantially free of, preferably doesnot comprise, nicotinic acid and salts thereof.

Preferred is a method of the present invention, wherein theblack-passivation composition is substantially free of, preferably doesnot comprise, phytic acid and salts thereof.

In most cases, preferred is a method of the present invention, whereinthe black-passivation composition is substantially free of, preferablydoes not comprise, intentionally added disulfides. This preferably meansthat the black-passivation composition is substantially free of,preferably does not comprise, intentionally added ionic disulfides andcompounds comprising a covalent disulfide. In turn, only in very rarecases a method of the present invention is preferred, wherein theblack-passivation composition comprises disulfides, preferably ionicdisulfides and/or compounds comprising a covalent disulfide. However, ingeneral, the absence of such compounds is generally preferred.

Preferred is a method of the present invention, wherein theblack-passivation composition is substantially free of, preferably doesnot comprise, intentionally added iron ions. If iron ions are present inthe black-passivation composition they are released/dissolved from thezinc-iron alloy upon utilizing the black-passivation composition. Aftersetting up a respective black-passivation composition no iron ions aretypically present. After starting the method of the present invention,the total concentration is very low. Upon utilizing theblack-passivation composition the total concentration typicallyincreases. Thus, preferably, iron ions are present in a totalconcentration of 1 g/L or below, based on the total volume of theblack-passivation composition, preferably of 0.8 g/L or below, mostpreferably of 0.5 g/L or below, after step (C) is carried out multipletimes.

Preferred is a method of the present invention, wherein theblack-passivation composition is substantially free of, preferably doesnot comprise, tungsten ions.

More preferred is a method of the present invention, wherein theblack-passivation composition is substantially free of, preferably doesnot comprise, tungsten.

Preferred is a method of the present invention, wherein theblack-passivation composition is substantially free of, preferably doesnot comprise, tungsten disulfide particles, preferably is substantiallyfree of, preferably does not comprise, tungsten disulfide.

Preferred is a method of the present invention, wherein theblack-passivation composition is substantially free of, preferably doesnot comprise, thiodiglycol.

Preferred is a method of the present invention, wherein theblack-passivation composition is substantially free of, preferably doesnot comprise, silicon dioxide, silicates, a silane, and a silanecontaining compound, preferably is substantially free of, preferablydoes not comprise, a silicon-containing compound. Only in very rarecases a method of the present invention is preferred, wherein theblack-passivation composition comprises a silane and/or a silanecontaining compound. It is assumed that in some cases the presence ofthe silane and/or the silane containing compound positively affects thecorrosion resistance of the black-passivation layer. However, in manycases an excellent corrosion resistance is already achieved even withoutthe presence of a silane and/or a silane containing compound.

Preferred is a method of the present invention, wherein theblack-passivation composition is substantially free of, preferably doesnot comprise, cobalt ions, preferably is substantially free of,preferably does not comprise, cobalt. The presence of cobalt becomesmore and more an environmental issue. Only in very few cases, a methodof the present invention is preferred, wherein the black-passivationcomposition comprises cobalt ions and/or cobalt containing compounds.The presence of cobalt typically increases corrosion resistance of aheat-treated substrate with the black-passivation layer obtained by themethod of the present invention.

Preferred is a method of the present invention, wherein theblack-passivation composition is further comprising

-   -   (iii) one or more than one species of halogen ions, and/or    -   (iv) one or more than one carboxylic acid and/or salts thereof.

Said halogen ions are typically the counter ions of the one or more thanone species of metal ions selected from the group consisting oftrivalent chromium, titanium, and zirconium.

Preferred is a method of the present invention, wherein the one or morethan one species of halogen ions have a total concentration from 1 g/Lto 18 g/L, based on the total volume of the black-passivationcomposition, preferably from 2 g/L to 15 g/L, even more preferably from3 g/L to 12 g/L, most preferably from 4 g/L to 10 g/L, even mostpreferably from 6 g/L to 9 g/L.

Preferred is a method of the present invention, wherein the one or morethan one species of halogen ions comprises chloride ions and/or fluorideions. In some cases, fluoride preferably also serves as complexing agentfor the metal ions, most preferably if the one or more than one speciesof metal ions is selected from the group consisting of titanium andzirconium. Bromide ions are preferably not comprised in theblack-passivation composition.

More preferably, the chloride ions have a total concentration from 1 g/Lto 18 g/L, based on the total volume of the black-passivationcomposition, preferably from 2 g/L to 15 g/L, even more preferably from3 g/L to 12 g/L, most preferably from 4 g/L to 10 g/L, even mostpreferably from 6 g/L to 9 g/L. Most preferably, in theblack-passivation composition chloride ions are the only species ofhalogen ions.

Said one or more than one carboxylic acid and/or salts thereof typicallyserve as complexing agents for said one or more than one species ofmetal ions selected from the group consisting of trivalent chromium,titanium, and zirconium, most preferably for metal ions of trivalentchromium.

Preferred is a method of the present invention, wherein the one or morethan one c comprises a dicarboxylic acid, a tricarboxylic acid, and/orsalts thereof.

A preferred dicarboxylic acid and/or salts thereof comprises a C2 to C6dicarboxylic acid and/or salts thereof, preferably oxalic acid, malonicacid, and/or salts thereof, most preferably oxalic acid and/or saltsthereof.

A preferred tricarboxylic acid and/or salts thereof comprises citricacid and/or salts thereof.

Preferred is a method of the present invention, wherein the one or morethan one carboxylic acid and salts thereof have a total concentrationfrom 0.5 mmol/L to 120 mmol/L, based on the total volume of theblack-passivation composition, preferably from 8 mmol/L to 105 mmol/L,even more preferably from 15 mmol/L to 90 mmol/L, most preferably from30 mmol/L to 80 mmol/L, even most preferably from 45 mmol/L to 70mmol/L. More preferably, the above total concentration applies with theproviso that the black-passivation composition comprises at least one ormore than one dicarboxylic acid and/or salts thereof.

Even more preferably, the oxalic acid and salts thereof have a totalconcentration from 0.5 mmol/L to 120 mmol/L, based on the total volumeof the black-passivation composition, preferably from 8 mmol/L to 105mmol/L, even more preferably from 15 mmol/L to 90 mmol/L, mostpreferably from 30 mmol/L to 80 mmol/L, even most preferably from 45mmol/L to 70 mmol/L. Most preferably, oxalic acid and salts thereof arethe only dicarboxylic acids and salts thereof in the black-passivationcomposition, preferably the only carboxylic acids and salts thereof inthe black-passivation composition.

In some cases a method of the present invention is preferred, whereinthe black-passivation composition comprises citric acid and/or saltsthereof, preferably in a total concentration from 0.5 mmol/L to 120mmol/L, based on the total volume of the black-passivation composition,preferably from 8 mmol/L to 105 mmol/L, even more preferably from 15mmol/L to 90 mmol/L, most preferably from 30 mmol/L to 80 mmol/L, evenmost preferably from 45 mmol/L to 70 mmol/L. In such cases, citric acidand salts thereof are preferably the only tricarboxylic acid and saltsthereof in the black-passivation composition, most preferably the onlycarboxylic acid and salts thereof in the black-passivation composition.

Preferred is a method of the present invention, wherein theblack-passivation composition is further comprising

-   -   (v) nitrate ions.

Nitrate ions preferably act as oxidizing agent in the black-passivationcomposition.

Preferred is a method of the present invention, wherein the nitrate ionshave a total concentration ranging from 0.1 g/L to 20 g/L, based on thetotal volume of the black-passivation composition, preferably from 0.4g/L to 15 g/L, even more preferably from 0.8 g/L to 11 g/L, mostpreferably from 1.2 g/L to 7 g/L, even most preferably from 1.7 g/L to4.5 g/L.

The Contacting with the Black-Passivation Composition (Step (c))

In step (c) the substrate is contacted with the black-passivationcomposition, preferably as described above, more preferably as describedabove as being preferred.

Preferred is a method of the present invention, wherein in step (c) theblack-passivation composition has a temperature in a range from 10° C.to 80° C., preferably from 15° C. to 65° C., even more preferably from19° C. to 45° C., most preferably from 22° C. to 38° C. If thetemperature is significantly exceeding 80° C., in many cases anundesired rapid dissolution (stripping) of the zinc-iron alloy isobserved impairing the corrosion resistance. However, if the temperatureis too low, in many cases the contacting in step (c) is undesirablylong.

Preferred is a method of the present invention, wherein in step (c) thecontacting is performed for a time period from 10 seconds to 200seconds, preferably from 20 seconds to 160 seconds, even more preferablyfrom 40 seconds to 130 seconds, most preferably from 60 seconds to 100seconds. If the time period is significantly exceeding 200 seconds, inmany cases an undesired rapid dissolution (stripping) of the zinc-ironalloy is observed impairing the corrosion resistance. However, if thetime period is too low, typically the blackening is insufficient andthus, the optical appearance is negatively impaired.

Preferred is a method of the present invention, wherein step (c) isperformed without applying an electrical current. Preferably, in step(c) the substrate is dipped into the black-passivation composition.

Post-Treatment

As shown in the examples below, an indeed very good corrosion resistanceis obtained if after step (c) the substrate is further treated in orderto increase corrosion resistance.

Generally preferred is a method of the present invention, wherein thesubstrate obtained after step (c) of the method of the present inventionis additionally treated with a post-dip composition and/or a sealercomposition, preferably as outlined below, either in this order or inreversed order.

Preferred is a method of the present invention, wherein step (c) isfollowed by step

-   (d) contacting the substrate obtained after step (c) with a post-dip    composition such that a post-dipped substrate is obtained.

Preferred is a method of the present invention, wherein the post-dipcomposition is acidic, preferably has a pH ranging from 3.0 to 6.8, morepreferably from 3.5 to 6.5, even more preferably from 4.0 to 6.3, mostpreferably from 4.3 to 6.0.

Preferred is a method of the present invention, wherein the post-dipcomposition comprises trivalent chromium ions.

Preferred is a method of the present invention, wherein the sealercomposition is substantially free of, preferably does not comprise,compounds and ions comprising hexavalent chromium.

Preferred is a method of the present invention, wherein the post-dipcomposition comprises phosphate ions.

Preferred is a method of the present invention, wherein the post-dipcomposition comprises one or more than one wetting agent.

Preferred is a method of the present invention, wherein the post-dipcomposition comprises one or more than one complexing agent, preferablyfor the trivalent chromium ions.

Preferred is a method of the present invention, wherein in step (d) thepost-dip composition has a temperature in a range from 18° C. to 60° C.,preferably 20° C. to 58° C., more preferably from 28° C. to 56° C., evenmore preferably from 33° C. to 54° C., most preferably from 38° C. to50° C.

Preferred is a method of the present invention, wherein in step (d) thecontacting is performed for a time period from 5 seconds to 200 seconds,preferably from 10 seconds to 140 seconds, even more preferably from 20seconds to 100 seconds, most preferably from 30 seconds to 70 seconds.

Preferred is a method of the present invention, wherein step (d) isfollowed by step

-   (e) contacting the substrate obtained after step (d) with a sealer    composition such that a sealed substrate is obtained.

Preferred is a method of the present invention, wherein the sealercomposition is alkaline, preferably has a pH of 9 or higher, morepreferably in a range from 9.1 to 12, even more preferably from 9.3 to11, most preferably from 9.5 to 10.5.

Preferred is a method of the present invention, wherein the sealercomposition comprises two or more than two organic compounds.

Preferred is a method of the present invention, wherein the sealercomposition is substantially free of, preferably does not comprise,trivalent chromium ions.

Preferred is a method of the present invention, wherein the sealercomposition comprises at least one organic polymer, preferablycomprising a polyurethane, a polyalkylene (preferably polyethylene), apolyfluoroalkylene (preferably polytetrafluoroethylene) and/or apolyacrylate.

Preferred is a method of the present invention, wherein the sealercomposition comprises one or more than one wax.

Preferred is a method of the present invention, wherein the sealercomposition comprises one or more than one silicon-containing compound,preferably at least one silane and/or at least one inorganic silicate.Preferably, the at least one inorganic silicate is a colloid.

Preferred is a method of the present invention, wherein in step (e) thesealer composition has a temperature in a range from 15° C. to 35° C.,preferably 17° C. to 30° C., more preferably from 19° C. to 27° C., mostpreferably from 21° C. to 25° C.

Preferred is a method of the present invention, wherein in step (e) thecontacting is performed for a time period from 5 seconds to 200 seconds,preferably from 10 seconds to 140 seconds, even more preferably from 20seconds to 100 seconds, most preferably from 30 seconds to 70 seconds.

When performing steps (d) and (e) as outlined above in this order, notonly a very good black-passivation layer is obtained but additionallyalso an excellent corrosion resistance, most preferably up to 480 hours,based on ISO 9227, 5% white rust limit.

Preferred is a method of the present invention, wherein step (e) isfollowed by step

-   (f) drying the substrate obtained after step (e).

Preferred is a method of the present invention, wherein step (f) iscarried out at a temperature ranging from 55° C. to 95° C., preferably58° C. to 90° C., more preferably from 58° C. to 85° C., most preferablyfrom 60° C. to 80° C.

Preferred is a method of the present invention, wherein step (f) iscarried out for a time period from 2 minutes to 20 minutes, preferablyfrom 3 minutes to 16 minutes, even more preferably from 4 minutes to 13minutes, most preferably from 6 minutes to 10 minutes.

Preferably, a drying step, preferably as defined above, is also carriedout after one or more than one of the previous steps, e.g. after step(c), step (d), etc. Very preferred is a method of the present invention,wherein a drying step, preferably as defined in step (f), is carried outafter step (d) and prior to step (e). This is very preferred because instep (d) the post-dip composition is acidic, wherein in step (e) thesealer composition is alkaline.

The present invention furthermore refers to a black-passivationcomposition for depositing a black-passivation layer on a zinc-ironalloy, the composition comprising

-   -   (i) one or more than one blackening agent selected from the        group consisting of    -   formula (I),

-   -   -   wherein        -   R¹ and R² are independently selected from the group            consisting of hydrogen and C1 to C5 alkyl,        -   R³ is selected from the group consisting of sulfonic acid,            carboxylic acid, alkyl carboxylic acid, phosphonic acid,            salts and esters thereof, and        -   n is 1, 2, 3, 4, or 5,

    -   and formula (II),

-   -   -   wherein        -   R⁴ is selected from the group consisting of sulfonic acid,            carboxylic acid, alkyl carboxylic acid, phosphonic acid,            salts and esters thereof, and        -   m is 1, 2, 3, 4, or 5, and

    -   (ii) one or more than one species of metal ions selected from        the group consisting of trivalent chromium, titanium, and        zirconium,

    -   with the proviso that        -   the black-passivation composition is substantially free of,            preferably does not comprise, nickel ions, cobalt ions, and            tungsten ions.

In particular preferred is a black-passivation composition of thepresent invention, wherein the black-passivation composition issubstantially free of intentionally added zinc ions, preferably does notcomprise intentionally added zinc ions. Most preferably, theaforementioned regarding zinc ions in view of the method of the presentinvention in particular applies likewise to the black-passivatingcomposition of the present invention.

Preferably, the aforementioned regarding the black-passivationcomposition utilized in the method of the present invention (inparticular what is defined as being preferred) applies likewise to theblack-passivation composition of the present invention. This applies inparticular to compounds and ions not contained in the black-passivationcomposition utilized in the method of the present invention.

The present invention furthermore refers to the use of one or more thanone blackening agent selected from the group consisting of

-   -   formula (I),

-   -   -   wherein        -   R¹ and R² are independently selected from the group            consisting of hydrogen and C1 to C5 alkyl,        -   R³ is selected from the group consisting of sulfonic acid,            carboxylic acid, alkyl carboxylic acid, phosphonic acid,            salts and esters thereof, and        -   n is 1, 2, 3, 4, or 5,

    -   and formula (II),

-   -   -   wherein        -   R⁴ is selected from the group consisting of sulfonic acid,            carboxylic acid, alkyl carboxylic acid, phosphonic acid,            salts and esters thereof, and        -   m is 1, 2, 3, 4, or 5,

    -   for blackening a zinc-iron alloy.

Preferably, the aforementioned regarding the one or more than oneblackening agent of formula (I) and (II) (in particular what is definedas being preferred) used in the black-passivation composition which isutilized in the method of the present invention applies likewise to theuse of the present invention.

The present invention is described in more detail by the followingnon-limiting examples.

Examples

In the examples, various test passivation compositions were preparedwith the numbering as introduced in Table 1 below, each composition isaqueous and generally comprises a species of metal ions; 6 g/L to 8 g/Lchloride ions if chromium ions were utilized; 50 mmol/L to 70 mmol/Loxalic acid if chromium ions were utilized or 10 mmol/L to 300 mmol/Lfluoride ions if titanium ions and zirconium ions were utilized,respectively, as complexing agents; approximately 1 g/L to 7 g/L nitrateions; and one of the following compounds abbreviated as below:

DPS: 3-(N,N-Dimethylthiocarbamoyl)-thiopropanesulfonate, sodium salt;also known as Raluplate DPS (CAS 18880-36-9; Raschig company); acompound of formula (I), wherein R¹ and R² are methyl, R³ is the sodiumsalt of sulfonic acid, and n is 3;

ZPS: 3-(2-Benzthiazolylthio)-1-propanesulfonate, sodium salt; also knownas Raluplate ZPS (CAS 49625-94-7; Raschig company); a compound offormula (II), wherein R⁴ is the sodium salt of sulfonic acid, and m is3;

SPS: Bis-(3-sulfopropyl)-disulfide, disodium salt; also known asRaluplate SPS (CAS 27206-35-5; Raschig company); comparative example;

SPV: 1-(3-Sulfopropyl)-2-vinylpyridinium betaine; also known asRaluplate SPV (CAS 90552-35-5; Raschig company); comparative example;

MPS: 3-Mercaptopropanesulfonate, sodium salt; also known as RaluplateMPS (CAS 17636-10-1; Raschig company); comparative example;

DTO: Dithiooxamide; comparative example.

The specific compound and its respective total concentration is outlinedin Table 1 below. Each test passivation composition has a pH ofapproximately 2.

In each example, as substrates a plurality of u-shaped iron platespecimens (base material) galvanized with a silver-like coloredzinc-iron layer (Hiron-Zn/Fe for high iron content and Protedur Plus forlow iron content, respectively, each is a product of Atotech; for ironcontent see Table 1 below) was dipped for approximately 90 seconds inthe respective test passivation composition, the compositions having atemperature of approximately 22° C. In examples according to theinvention, a blackening was immediately observed.

Afterwards the treated specimens were dried, and a post-dip composition(acidic, comprising trivalent chromium phosphate) as well as a sealercomposition (alkaline, comprising a wax and a silicon-containingcompound) was applied. Subsequently, the blackening quality was visuallyevaluated; the corrosion resistance according to ISO 9227.

TABLE 1 % blackening Species of Blackening corrosion* No. Fe agent[mmol/L] metal ions [g/L] quality [h] I-1 1.6 DPS 1.9 Cr(III) 3-4^(##) +<72 I-2 15 DPS 3.8 Zr(IV) 0.5 ++ <24 I-3 15 DPS 1.9 Ti(IV) 0.3 +++ 120I-4 15 DPS^(#) 1.9-8^(##) Cr(III) 3-4 +++ 480 II-1 15 ZPS 32.0 Cr(III)3-4 ++ 240 C1 15 SPS 14.1 Cr(III) 3-4 − n.d. C2 15 SPV 22.0 Cr(III) 3-4− n.d. C3 15 MPS 28.1 Cr(III) 3-4 − n.d. C4 15 DTO 41.6 Cr(III) 3-4 n.d.n.d. “*” denotes: time until max. 5% white rust was observed, “^(#)”also tested on screws in barrel applications, “^(##)” tested incompositions with various concentrations, “n.d.” denotes not determined,typically because of no (sufficient) blackening, “+++” denotes anexcellent blackening; no visual defects are noticeable all over thespecimen, “++” denotes still an acceptable blackening with only minorbut still acceptable visual defects; the whole specimen is sufficientlyblack, “+” denotes not anymore acceptable blackening with significantvisual defects and areas of the zinc-iron layer, “−” denotes no oralmost no blackening, visual defects and large areas of the zinc-ironlayer

In all examples according to the invention (i.e. examples I-1, I-2, I-3,I-4, and II-1) a blackening of a zinc-iron alloy was obtained. Very goodresults were obtained in examples I-3 and I-4, wherein example I-4provided these very good results even over a comparatively wideconcentration range for DPS and chromium ions.

In contrast, alternative compounds (comparative examples C1-C3) provideno or almost no blackening of the zinc-iron alloy. Example C4 revealedthat DTO was not soluble at all and, thus, could not be used for testingand is found unsuitable. If no blackening was obtained, corrosionresistance was not further tested because blackening was a basicrequirement.

In further comparative examples, above test passivation compositions (inparticular according to the present invention) were tested with saidsubstrates but having a zinc (no zinc alloy) or a zinc-nickel alloylayer thereon. In each further example no blackening was obtained (i.e.evaluated as “˜”). As a conclusion, the test passivation compositionsutilized in the method of the present invention very specificallyblacken zinc-iron alloys.

1. A method for forming a black-passivation layer on a zinc-iron alloy,the method comprising the steps: (a) providing a substrate comprisingthe zinc-iron alloy, (b) providing a black-passivation composition fordepositing the black-passivation layer on the zinc-iron alloy, thecomposition comprising (i) one or more than one blackening agentselected from the group consisting of formula (I),

wherein R¹ and R² are independently selected from the group consistingof hydrogen and C1 to C5 alkyl, R³ is selected from the group consistingof sulfonic acid, carboxylic acid, alkyl carboxylic acid, phosphonicacid, salts and esters thereof, and n is 1, 2, 3, 4, or 5, and formula(II),

wherein R⁴ is selected from the group consisting of sulfonic acid,carboxylic acid, alkyl carboxylic acid, phosphonic acid, salts andesters thereof, and m is 1, 2, 3, 4, or 5, and (ii) one or more than onespecies of metal ions selected from the group consisting of trivalentchromium, titanium, and zirconium, and (c) contacting the substrate withsaid black-passivation composition such that the black-passivation layeris formed on the zinc-iron alloy.
 2. The method of claim 1, wherein inthe zinc-iron alloy the amount of iron ranges from 0.1 wt.-% to 30wt.-%, based on the total weight of the zinc-iron alloy.
 3. The methodof claim 1, wherein the black-passivation composition is acidic.
 4. Themethod of claim 1, wherein in the black-passivation composition the oneor more than one blackening agent has a total concentration ranging from0.2 mmol/L to 100 mmol/L, based on the total volume of theblack-passivation composition.
 5. The method of claim 1, wherein theblack-passivation composition comprises one or more than one blackeningagent of formula (I) in a total concentration ranging from 0.4 mmol/L to25.0 mmol/L, based on the total volume of the black-passivationcomposition.
 6. The method of claim 1, wherein R¹ and R² areindependently selected from the group consisting of hydrogen, methyl,ethyl, 1-propyl, 2-propyl, 1-butyl, 2-butyl, and tert-butyl.
 7. Themethod of claim 1, wherein R³ comprises at least sulfonic acid, saltsand/or esters thereof.
 8. The method of claim 1, wherein R⁴ comprises atleast sulfonic acid, salts and/or esters thereof.
 9. The method of claim1, wherein said one or more than one species of metal ions has a totalconcentration ranging from 0.1 g/L to 30 g/L, based on the total volumeof the black-passivation composition and based on the mass of the freeions.
 10. The method of claim 1, wherein the black-passivationcomposition is substantially free of, preferably does not comprise,tungsten.
 11. The method of claim 1, wherein the black-passivationcomposition is further comprising (iii) one or more than one species ofhalogen ions, or (iv) one or more than one carboxylic acid and/or saltsthereof, or both (iii) and (iv).
 12. The method of claim 1, wherein instep (c) the black-passivation composition has a temperature in a rangefrom 10° C.
 13. The method of claim 1, wherein in step (c) thecontacting is performed for a time period from 10 seconds to 200seconds.
 14. A black-passivation composition for depositing ablack-passivation layer on a zinc-iron alloy, the composition comprising(i) one or more than one blackening agent selected from the groupconsisting of formula (I),

wherein R¹ and R² are independently selected from the group consistingof hydrogen and C1 to C5 alkyl, R³ is selected from the group consistingof sulfonic acid, carboxylic acid, alkyl carboxylic acid, phosphonicacid, salts and esters thereof, and n is 1, 2, 3, 4, or 5, and formula(II),

wherein R⁴ is selected from the group consisting of sulfonic acid,carboxylic acid, alkyl carboxylic acid, phosphonic acid, salts andesters thereof, and m is 1, 2, 3, 4, or 5, and (ii) one or more than onespecies of metal ions selected from the group consisting of trivalentchromium, titanium, and zirconium, with the proviso that theblack-passivation composition is substantially free of nickel ions,cobalt ions, and tungsten ions.
 15. (canceled)
 16. The method of claim1, wherein in the zinc-iron alloy the amount of iron ranges from 6.1wt.-% to 15 wt.-%, based on the total weight of the zinc-iron alloy. 17.The method of claim 1, wherein formula (I) comprises the compound3-(N,N-Dimethylthiocarbamoyl)-thiopropanesulfonic acid, salts and/oresters thereof.
 18. The method of claim 1, wherein formula (II)comprises the compound 3-(2-Benzthiazolylthio)-1-propanesulfonic acid,salts and/or esters thereof.
 19. The method of claim 1, wherein in thezinc-iron alloy the amount of iron ranges from 2.1 wt.-% to 25 wt.-%,based on the total weight of the zinc-iron alloy, the black-passivationcomposition has a pH from 1.0 to 4.5, in the black-passivationcomposition the one or more than one blackening agent has a totalconcentration ranging from 0.2 mmol/L to 100 mmol/L, based on the totalvolume of the black-passivation composition, and said one or more thanone species of metal ions has a total concentration ranging from 0.1 g/Lto 30 g/L, based on the total volume of the black-passivationcomposition and based on the mass of the free ions.